Power amplifiers are categorized into several classes of operation. Some of these classes include Class-A, Class-B and Class-AB. A Class A power amplifier is defined as an amplifier with or without negative feedback, and in its ideal case is characterized with the greatest fidelity in faithfully amplifying an input signal with the least distortion. It conducts output current throughout 100% of the input signal waveform. In other words it exhibits a conduction angle of exactly 2π radians. In most cases it exhibits the greatest gain of all power amplifier classes. Another characteristic of the Class A amplifier is that its DC bias point is generally selected to be at ½ the transistor's peak current capability and ½ its peak voltage capability. It is however the least efficient of all the classes of amplifiers, in as much that in ideal cases the power delivered to the load is typically only 50% of the D.C. power used.
A Class AB amplifier is defined in the ideal case as a power amplifier that has an output current flow for more than half, but less than all, of the input cycle. In other words it exhibits a conduction angle between π and 2π radians. See, e.g., Gilbilisco, Stan, Ed. Amateur Radio Encyclopedia, TAB Books, 1994. Another characteristic of a Class AB amplifier is that it is generally biased at less than ½ of the transistor's peak current capability. The advantages of a Class AB amplifier include improved high power efficiency over a Class A type amplifier and improved efficiency at low drive levels. The drawbacks of Class AB power amplifiers include the generation of an output signal which is not an exact linear reproduction of the input waveform, lower gain than that of a Class A type, continuous current drain, and lower efficiency compared to other amplifier types.
An ideal Class B amplifier is defined as an amplifier that has output current flow for ½ the cycle of the input signal wave form. In other words it exhibits a conduction angle of π radians. The advantages of a Class B amplifier are improved high power efficiency over Class A and AB type amplifiers and improved efficiency at low drive levels. The drawbacks of a Class B amplifier include even higher distortion and lower gain compared to Class A and AB amplifiers.
Class A, AB and B amplifiers are typically used in the transmitters of cellular mobile terminals. The Class selected is often dictated by the communications standard employed by the terminal. Typically a GSM type handset will utilize a Class B amplifier stage as the final amplifier of the transmitter. This is because the GMSK standard employed in a GSM handset embeds the voice or data being transmitted in the phase angle of the signal, which is sometimes referred to as a constant envelope signal. Such signals are more tolerant to amplitude distortion during the amplification and transmission process. One benefit of using a Class B amplifier in these handsets is that it results in longer battery life and thus longer talk times.
In an NADC or CDMA cellular mobile terminal the final amplifier in the transmitter chain is typically a Class AB type amplifier. This is because the data or voice being transmitted is encoded in both the amplitude and phase of the signal. This results in a signal with a non-constant envelope, requiring a transmitter having a minimal amount of both amplitude and phase distortion.
Although Class AB operation improves power efficiency at the cost of some linearity of signal amplification, it has become the amplifier class of choice for non-constant envelope mobile cellular and PCS transmitters. Unlike the Class B type amplifier, the Class AB requires a quiescent current bias. And although it exhibits better efficiency at low drive levels than a Class A type amplifier, the optimum low drive efficiency is limited by the linearity requirement under higher drive. In general, as power efficiency improves under high drive, linearity will suffer. The inverse relationship of efficiency at high drive, low quiescent bias, high efficiency at low drive and the need for high drive linearity makes the selection of a quiescent bias point a critical design parameter. The operation of the Bipolar or FET transistors at low quiescent bias points also exposes the amplifier to greater variability in performance at both low and high operating temperatures.
Ideally, a power amplifier for use in mobile terminal equipment such as cellular or PCS communication devices employing a non-constant envelope modulation scheme should amplify the input signal linearly with minimal distortion of the signal and with optimum efficiency across a wide range of drive levels.
In practice, balancing linearity and efficiency across all drive conditions is difficult. Accordingly, a need exists for an improved biasing arrangement that dynamically adjusts the operating mode of the power amplifier as a function of drive signal, temperature or other relevant factors. Such a biasing arrangement would both eliminate excessive power dissipation in the output stages at low drive levels and minimize distortion over a broad range of drive levels.